1. Technical Field
This invention relates to controlling the flow of current to the coil of a solenoid.
2. Background Art
Various circuitry for driving solenoids is known. For example, it is known to apply a driving current to a solenoid in accordance with a periodic function, such as a square wave, thus energizing the solenoid with an average current less than the maximum applied current. It is also known that after a solenoid is energized and initial displacement has taken place, a reduced amount of power is necessary to maintain the solenoid in an energized condition. Thus, it is possible to reduce power consumption in a solenoid by initially applying a higher peak current magnitude and then reducing the current to a lower sustaining value. Such a current reduction can take place, for example, after a certain amount of time has passed. It may be desirable to vary the magnitude of the average solenoid current so that the solenoid can provide a variable force.
Specific examples of circuitry for driving solenoids include U.S. Pat. No. 4,180,026 to Schulzke et al which teaches a pair of transistors to drive a solenoid. One of the transistors is turned on only between driving periods. Solenoid driving circuits with two transistors are also taught in patents to Ohba, U.S. Pat. Nos. 4,347,544 and 4,360,855. U.S. Pat. No. 3,581,156 to Dolbachian et al teaches an electromagnetic clutch driver having switches by which the clutch coil can be driven in a variety of modes. U.S. Pat. No. 4,327,394 to Harper teaches a relatively slow decay from a peak current to a sustaining current.
In particular, it is known to use a switching coil driver to control current to automotive fuel injector and transmission solenoids and to use switching (on-off) techniques to both minimize power dissipation and, in some cases, minimize solenoid non-linearity and hysteresis.
A solenoid driver may supply current to the coil as a current sinking or a current sourcing device. As a current sinking device, one side of the solenoid coil is connected to the battery. The solenoid is turned on by grounding (sinking) the other side of the coil through a switch such as a transistor. As a current sourcing device, one side of the solenoid coil is connected to ground. The solenoid is turned on by connecting the other side of the coil to battery voltage through a switch controlled by the solenoid driver. This configuration has the advantage of protecting for an accidental short to ground in the wiring harness between the solenoid driver and the solenoid. If this happens the solenoid will turn off rather than on, as would happen with the current sinking configuration. Turning the solenoid off is a preferred failure mode since it is advantageous to have the primary failure mode (open electrical connection) the same as the secondary failure mode (short to ground). Both configurations have the advantage of requiring only one wire from the driver to the solenoid.
A publication by SGS-ATES Semiconductor Corporation in June 1982 entitled "Injector Driver Control--Tentative Data Sheet" discloses a current sinking device with a series transistor controlling flow through a solenoid coil and a sensing resistor. A second transistor selectively provides a current path parallel to the solenoid coil. The two transistors are controlled to reduce solenoid current from an initial peak current to reduced magnitude sustaining currents. However, independent adjustment of the various current levels is not taught.
Even though reducing solenoid driving current from a peak current to a sustaining current is known, it is still desired to obtain a means to selectively control the solenoid driving current so as to provide control of solenoids providing a force or a position linearly proportional to applied solenoid coil current. In particular, an improved logic for controlling solenoid driving current would be desirable. This would improve efficiency and make the solenoid output force more responsive to desired changes. These are some of the problems this invention overcomes.